Roll Camera Following a Path
Instead of having the camera track anything, let an empty do the job. Parent the tracker empty to the path and let it track the focus empty, then parent the camera to the tracker empty. That way you can rotate the camera freely. All the path motion is done by lesser beings.

In the picture I have moved the camera a little away from the empty to show the relationship line. That's just for clarity purposes. Usually you'd keep it smack on the tracker empty.

Another benefit: since the camera is in local space due to parenting it, you can lock its X and Z rotation. 



The empties do all the pointing and moving, so rolling the camera around its local Y axis is all that's left to do.



Inbetween objects simplify a lot of complex hierarchies.